This invention relates generally to image scanners. More specifically, one aspect of the present invention relates to an improved scan window in a scanner, and another aspect of the present invention relates to an improved configuration for scanners employing a fluorescent tube.
FIG. 1 is a cross-sectional view of an existing hand-held scanner 100. It includes a housing 102 with a scan window 104. An LED bar 106, a reflective element 108, a lens assembly 110 and a sensor 112 provide the scanning mechanism for the scanner 100. A medium to be scanned, typically a sheet of paper 120 having a surface 122 upon which an image resides, lies beneath the scan window 104.
In operation, the LED bar 106 emits light. The emitted light passes through a first facet 130 of the scan window 104, strikes the surface 122 of the medium 120 and is reflected. Some of the reflected radiation passes through a second facet 132 of the scan window 104 and strikes the reflective element 108. The reflective element 108 directs the reflected light passing through the second facet 132 to the sensor 112 after passing it through the lens assembly 110. An image on the surface 122 of the medium 120 is detected by the sensor 112 through variations in reflected light intensity as the sensor is moved in the direction indicated by arrow 140, as is well known in the art.
The hand scanner 100 has a number of disadvantages. One difficulty with hand scanner 100 is that the lens assembly does not have a uniform responsiveness across a scan width. The scanner 100 typically scans about four inches at one time. The LED bar 106 has a plurality of LED elements arrayed along this width, each emitting light. The lens assembly 110 tends to be more responsive to reflected light in the middle of the scan than towards the edges of a scan, meaning that images are both artificially brighter in the middle and artificially darker at the edges. This is not desirable in scans producing gray scale or color equivalents of an image. One solution to this problem has been to tailor an output profile of the LED bar 106 to compensate for the response of the lens assembly 110. First, the transmission profile of the lens assembly, such as shown in FIG. 2A, is determined. Next, a desired LED output profile is computed as the inverse of the lens assembly transmission profile, such as shown in FIG. 2B. The LED bar 106 output is then tailored to the computed profile by use of resistors, for example.
One important consideration in image scanning is to achieve maximum intensity of incident light to provide sufficient contrast for image development. For this reason, the first facet 130 and the second facet are made transparent to reduce attenuation of the light from the LED bar 106. However, foreign deposits on the scan window can obscure the image. The second facet 132 is raised from the area of the medium surface 122 being scanned. This can be alleviated, as shown in FIG. 1, by the shape of the scan window 104. The lens assembly 110 has a focal point at the surface 122. The raised position of the second facet 132, placing it outside the focal point, decreases incidences of false images due to foreign matter deposited on the scan window 104.
Another potential source of image degradation is from the individual LEDs elements of the LED bar 106. FIG. 3 is a view of the LED bar 106 comprising several LEDs 150. As shown, each LED 150 has a light-emitting element 152 encased in a protective lens 154. Each LED 150 emits light in a conic illumination field 160. As a distance from the LED bar 106 extends outward, the conic illumination fields 160 from the several LEDs 150 begin to converge and overlap. Desirably the paper surface is positioned at that distance from the LED bar 106 where the illumination fields begin to overlap, as shown at position 200 in FIG. 3. If the paper surface 122 is too close, position 202, or too far away, position 204, image artifacts can be produced. The artifacts develop from the variation of the incident radiation on the image. It is difficult to accurately position and maintain the paper surface at the desirable distance from the LED bar 106.
Another drawback to the use of LEDs arises in the scanning of color images. Because LEDs generally produce monochromatic light, a scanner employing LEDs for illumination will be partially "colorblind". To avoid this problem, some scanners illuminate the matter to be scanned by fluorescent tubes, which produce light having a broadband frequency distribution.
Not without their drawbacks as well, though, fluorescent tubes require a socket at each end, creating an illumination "dead space" at either end of the tube, as illustrated in FIG. 4. Additionally, the illumination provided by a fluorescent tube is dim in the regions adjacent the sockets. This increases the effective illumination dead space, which necessitates the scanning device being wider than the image field to be illuminated. These problems with fluorescent tubes can be particularly problematic when scanning materials such as books. When scanning a page on one side of a book, the pages on the other side typically arc upwards, and for a scanner wider than its field of illumination, can prevent the scanner from properly scanning the centermost edge of text. Prior art techniques for minimizing this problem typically involve attempting the minimize the width of the fluorescent tube sockets. However, the technical limits to this approach make it an unsatisfactory solution for the physical drawbacks of fluorescent tubes.